Nucleation of Manganese Oxides in the Presence of Reactive Halogen Species

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Funded by the Environmental Chemical Sciences Program of the NSF Division of Chemistry, Professor Young-Shin Jun at Washington University examines the formation of manganese oxide particles in salt water. Manganese oxides are environmentally abundant and are important in removing contaminants. Their formation can be affected by products from human activities that have high salt concentrations, such as from desalination and oil and gas recovery. The chemical reactions involved are poorly understood, including the roles of halide ions (e.g., chloride and bromide) and highly reactive halogen species. This work quantitatively and qualitatively examines how salt water chemistry affects the formation of manganese oxides in the environment. The project also develops environmental chemistry outreach programs for underrepresented or economically disadvantaged K-12 students. This outreach both encourages the early involvement of high school and undergraduate students in research and improves university courses by incorporating research outcomes.The increased production of highly saline effluents from water and energy generating processes raises questions about their impacts on the environment. In particular, halides in brine can greatly affect mineral formation and dissolution processes in natural aquatic systems. With sunlight, halides may further undergo photochemical reaction to generate reactive radical species. However, the effects of halides under sunlight exposure on redox-active transition metal ions and their solid phase formation are still obscure. This project investigates the effects of reactive halogen species on the oxidation of aqueous manganese (Mn) ions and the kinetics of the nucleation and growth of Mn particles, with and without natural organic matter. It is hypothesized that reactive species generated from reactions between natural organic matter and halides during their photolysis can promote the photochemical oxidation of Mn2+ (aq) and facilitate the formation of higher oxidation states of manganese oxides. To elucidate the properties of newly formed manganese oxides and to examine these dynamic redox reaction behaviors, solid phases of manganese oxides and fluid chemistries are characterized. In situ synchrotron-based small angle x-ray scattering provides real-time information about the nucleation and growth of manganese oxides. The project advances fundamental knowledge regarding the nucleation mechanisms and kinetics of manganese oxides in highly saline systems, which is important for predicting contaminant removal and transport and for developing new manganese-based materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.